Control apparatus



9 H. s. JONES 2,390,793

' poNTRbL APPARATUS Filed June 13, 1942 3 Sheets-shat 1 H-ll II II IHARRY S- JONES A ORNEY 3 Sheets-Sheet 2 R. HARRY. s, JONES FIG.2.

H. S. JONES CONTROL APPARATUS Filed June 13, 1942 Dec. 11, 1945.

can B JW% 7 RNEY CONTROL APPARATUS Filed June 13 1942 v 3 Sheets-Sheet 3mvsmox. HARRY s. JONES A ORNEY Patented Dec. 11, 1945 UNITED STATESPATENT OFFICE M s. 217 3131 32? m t.

The Brown Instrument Company,

Philadelphia,

Pa., a corporation of Pennsylvania Application June 13, 1942, Serial No.448,946

9 Claims. (Cl. 172-239) The general object of the present invention isto provide improvements in electrical control systems oi thepropontio'ning type.

More specifically stated, the general object of the present invention is.to increase the flexibility and capacity for adjustment of a controlsystem of the type specified, by combining therein a plurality ofseparate bridge circuits, two diiierent points oieach of which areconnected into a control circuit through which the operation of areversible electric motor is controlled to produce suitable controlactions, such, for example, as the adjustments of the fuel valve of aheating furnace. By thus combining a plurality of bridge circuits in myimproved control system I obtain the advantage that sensitivity and/orother adjustments of one bridge circuit may be made without directlymodifying the condition oi any other bridge circuit, or the relation oftbs; latter to other portions oithe control sys- The various features ofnovelty which characterise my invention are pointed out withparticularity in the claims annexed to and forming a part of thisspecification. For a better understanding of the invention, however, itsadvantages and specific objects attained with its use, reference shouldbe had to the accompanying drawlugs and descriptive matter in which Ihave illustrated and described preferred embodiments of the invention.

oi the drawings:

,8 opening andclosing adjustments as required to maintain anapproximately constant furnace temperature condition.

A change in the furnace temperanire producesachangeinlmlbpressureinthebulbl which is transmitted to a Bourdon tubeG. The expension and contraction oi the tube Cngives correspondingmovement to a contact H which is thereby adjusted along the length of aresistor included in a Wheatstone bridge I. The latter includes anotherresistor along which a contact J is adjusted by and in 'accordance withthe movements of the control motor C. The expansion and contraction ofthe Bourdon tube G also gives movements to a contact K, and thereby, asis hereinafter explained, controls the operation of a reversibleelectric reset motor L. The movements of the latter give correspondingmovements to a contachM thereby adjusted along the length of a resistorincluded in a. second bridge circuit IA.

As shown in Fig. 1, the two bridge circuits I and IA are energized byalternating current supply conductors I and 2 through a, transformer swhich includes a primary coil 3 and two second- Flg. 1 is a diticrepresentation of a control system;

Fig. 2 is adiagram illustratinflan electronic- A is supplied with fluidfuel at a rate dependent uponadjustmentoiatuelvalveB. Thelatterismechanicallyconnected to and adjusted by the rotative movements of areversible electric con- The operation of the motor C representation ofa v ary coils 4 and 5. Primary coll I is connectedacross the supplyconductors l and 2. The secondary coil 4 is the direct energizingelement of the bridge I which includes resistors} and i connected inparallel with one another to the terminals of the coil 6. The resistor 6is engaged at a variable point along its length by the slider contact'H,and the resistor l is engaged ata variable point along its length by theslider contact .1. A resistance 8 is connected in shunt to theintermediate portion of the resistor I along which thelcontact J moves.Associated with the resistance 8 is a contact 8 manually adjustable toshort circuit and render inoperative, more or less of the resistance 8,and thereby vary the so called throttling range oi 'the control system.

The secondary transformer coil I forms the direct energizing element orthe bridge circuit IA which includes resistors II and H. Those resistorsare connected between the terminals of the coil I in parallel with oneanother and each in series with a resistance 12. The previouslymentionedeontactllisadiustedbythew rationoitheresetmotorlialongtheresistorll. A contact it manually adimtablealong the resistance]! forms a means iorshortcircuitingot that redstmcel2, and thereby regulating the ratooirelmelloctedbythemotoriifinenauwwnenu other shown in detail in Fig. 2, and which is of known type.comprises-input terminals E, energizing terminals E, and outputterminals E One input terminal E, is connected by a conductor 15 to thecontact H, while the other is connected by a conductor It to themidpoint lid of the resistor H. A conductor l1 connects contacts J andM. The inputterminals E Of-the amplifier E are thus connected in serieswith the movable contacts H, J and M and resistor midpoint lid in acontrol circuit, associated with the two main resistors of each of thetwo bridges Iand IA. One or the energizing terminals E is connected tothe supply conductor l, and the g is connected to the supply conductor2. As diagrammatically shown, the control motor Dis a reversible twophase motor having one put terminal E 20' connected nected to thewindings. 33 and 23, respectively, through limit switches 3' and 6'. Oneor the other or said limit switches is opened by an arm 0 rotated by themotor 0 when the valve 8 is given the maximum desired closing oropeningadjustment. As the Bourdon tube G departs from its new tralposition as a result or the attainment of a subnormal temperature by thebulb F, the contact K engages a stationary contact 3|. The latter isconnected by a conductor 32 to one terminal of the winding 33 oi thereset motor L. 0onversely, when the bulb temperature rises above .thenormal, the contact K is moved into engagement with a second stationarycontact 33. The latter is connected by a conductor 36 to one terminal ofthe second winding 33 of the motor L.

winding 2| connected between the amplifier outand 2. 0n the adjustmentof the contact H away from its neutral or intermediate position as-thefurnace temperature to which the bulb F responds rises above or fallsbelow a predetermined normal value, a current flow is set up in thecontrol circuit including the amplifier in-.

put terminals E. 'The phase oi the control circuit current fiow will lagbehind or lead the phase of the voltage between the supply conductors land 2 approximately 90 accordingly as the contact H is displaced fromits neutral position in one direction or the other. when the'phase inthe control circuit lags or! the current fiow behind the supply circuitvoltage, the windings 2l'and 2l' co-operate to give a turning movementto the rotoroi' the motor D in a direction which is opposite to that ofthe movement given the rotor when the phase of the current flow in thecontrol circuit leads the supply voltage.

The control motor D is biased to an intermediate position, from which itis adapted to make a slight turning movement in one direction or theother and thereby angularly adjust a mercury switchll in a clockwise orcounterclockwise direction from theopen or neutral position or theswitch. The motor D and switch 2| need not be further described hereinas they form no part or the present invention and are oi types fullydisclosed in my turned clockwise out 01' its neutral position, which isthat shown in Pig. 1, it connects conductors 22 and 23. The conductor 22is connected to the supply conductor 2 and the conductor 23 is connectedto one terminal or the winding 24 of the motor C. On a counter clockwiseadjustment of the switch 2| away from its neutral podtion, the switchconnects the conductor -22 to a conductor 2| which is connected to aterminal'oi' the second motor winding 26. The second terminals of thewindings 24 and 26 are connected by conductor 21 to the supply conductorI.

It on an increase above normal 01 the temperature of the thermometerbulb F, the control motor D is actuated in the direction to give aclockwise adiustment to the switch 2|, thereby encruising the winding,the motor C should then operate in the direction to give a closingadjustment to valve 8 and thus reduce the heat supply 7 the valve 3. Theconductors 23 and 26 are conprior Patent 2,246,686,- granted June 24,1941. 'When the switch 2| is The second terminals of the windings 33 and38 are connected by a conductor 31 to the supply conductor I, and'thecontact K is connected by a conductor to the supply conductor 2. Limitswitches O and 0 included in series with the motor windings 33 and 36,.respectively, are opened by an arm 0A rotated by the motor L, when oneor the other end of the desired range of reset adjustment is reached.

As previously indicated, the amplifier E may be of any known or suitabletype. and, as shown,

it is of a t and form fully described in my prior Patent No. 2,246,686,granted June 24, 1941. As diagrammatically illustrated in Fig. 2, theamf plifier E includes energizing provisions comprising a transformer Phaving its primary coil connected through the amplifier terminals iii tothe supply conductors l and 2, and having a plurality orsecondarywindings P, P, and P The secterminals It across ondary windingP. supplies filament current to the electronic amplifier valves. Thesecondary winding P is connected to the anodes of a full waverectifying. tube Q. The secondary winding P supplies filament current tothe rectifier e tube Q. v

The amplifier E shown in Fig. 2 comprises an electronic valve R or theheater type pentode, operatively connected to the amplifier inputterminals E bymeans of a voltage divider including a resistor 46 and asliding contact 4| adjustable along the latter to vary the sensitivityof the control of the motor 0. The output circuit of the valve R isresistance-capacity coupled to the input circuit or a second electronicvalve 8, also shown as or the heater type pentode. The valve 8 has itsoutput circuit connected by a transformer T to the input circuits ofsimilar electronic valves U and V, each of which is of the heater typetetrode, and preferably of the type known commercially as beam-poweramplifier tubes. The valves U- and V, preferably and as shown, areconnected in push pull. The anodes or the valves U and V are'connectedto the terminals or the primary of a transformer W, the secondary ofwhich is connected to and energizes theampiifier output I which the coil23 of the. motor Dis connected.

Inasmuch as the amplifier arrangement shown in Fig. 2 is identical withan amplifier arrangement illustrated and described in detail in my.prior Patent No. 2,246,686, further description herein of the amplifierE is unnecessary.

In the operation or the control apparatus shown in Fig. 1, the contact Jand H which form the output terminals of the bridge circuit 1, lm-, poseapotential on the control circuit in which they are included only whenthe potentials 01' said contacts are 'difierent. The potential impressedon the control circuit by contacts J and II will be in one direction orthe other, accordingly as the potential of the contact H is higher orlower than the potential of the contact J. Similarly the midpoint iia ofthe resistor ii and the contact M which form the output terminals of thebridge circuit IA, impress a potential on the control circuit in onedirection or the other when the potential of the contact Misrespectively greater or lessthan the potential at point Ila, andimpress no potential. on the control circuit when the point I! andcontact M are at the same potential.

When the furnace heat requirement is such that the temperature of thebulb F is maintained at its normal, or control point, value with thefuel valve B in its intermediate adjustment position, the potentialofthe contact J is the same as the potential of the contact H, and thepotential of the contact M is the same as the potential of the pointIla. with the control system thus balanced under the ideal or averageload condition, no potential is impressed on the control circuit by theoutput terminals of either of the bridge units I'and IA.

On an increase in the furnace heat requirement and the resultantdecrease of the temperature of the bulb 1' below its normal value, theBourdon tube G lowers the contacts H and K and this down movement of thecontact H creates a potential difference between the output terminals ofthe bridge I andthereby creates a current flow in the control circuitincluding the amplifier input terminals E. This results in a currentflow in the output circuit of the amplifier E which includes the winding20 of the motor D and causes that motor to eflect an adiustment of theswitch 2| which results in the operation of the motor C in the directionto give an opening adjustment to the valve B, and to lower the contactJ. The movements of the motor 0 and contact J thus initiated, continueuntil the system is rebalanced and current ceases to flow in its controlcircuit, or until the limit switch 0 is opened.

But for the operation of the reset motor L, the control system would berebalanced and the operation of the motor C would be interrupted as soonas the down movement of the contact J makes thepotential of the latterequal to that of the contact H. At that time assuming no change in theload requirement in the meantime, the temperature of the bulb F wouldnecessarily have a value slightly lower than its normal value. However,the reset motor L which is started into operation as soon as the contactK engages contact II, then operates to adjust the contact M in thedownward direction and thereby creates a potential diilerence betweenthe output terminals Ho and M of the bridge circuit IA, which maintainscurrent flow in the control circuit, and thereby continues the operationof the motor 6. when the potential of. the contact J becomes equal tothe potential of the contact H. The resultant further down movement ofthe contact J creates a potential difference between that contact andthe contact H which is opposite in direction in its eiiect on currentflow in the control circuit, to the potential difference between point Ila and contact M. In practice the system may be proportioned so that theeflect on the current flow inthe control circuit of the-movements of thecontacts J and M occurring after the contact J reaches the level of thecontact H, will neutralize one another.

Under the normal operating condition in which the furnace heatrequirement is within the operative range of the fuel valve adjustmenteflected by the motor C, the temperature of the bulb P will eventuallybe restored to its normal value and the contacts H and K will .bereturned to their neutral positions, thereby de-energizing the resetmotor L. When the operation of the reset motor is thus interrupted, thecontacts J and M will each be displaced from its neutral by an amountproportional to the difference between the actual furnace heatrequirement and the ideal or aver. age heat requirement which permitsthe temperatur of the bulb F to be maintained at its normal value withthe fuel valve B in is intermediate position.

The general cycle of control operations described above will be repeatedon any subsequent departure.of the temperature of the bulb 1" from itsnormal value. when that departure is a temperature increase, thecontacts H and K move upward from their neutral ositions, and theresulting adjustments of the contacts J and M will also be in the upwarddirection.

While the general operative results above described are obtainable withan electrical proportioning control system including a reset motor and acontrol motor associated with a single bridge circuit, the combinationof two bridge circuits such as the circuits I and IA, in the controlsystem, permits of desirable independent adjustments of the two bridgecircuits. Thus, with the particular arrangement shown in Fig. 1, thethrottling range adjustment of the control systern may be varied byadjustment of the contact 9 of the bridge circuit I without disturbingthe reset rate, and the adjustment of the contact I! of .the bridgecircuit IA, respectively decreases or increases the rate of reseteilected by the motor L without interfering with or modifying thethrottling range adjustment.

In Fig. 3 I have illustrated a control system like that shown in Fig. 1,in that it includes a fuel valve 3 for a furnace or heater A, and areversible control motor 0 which adjusts the valve B in response toadjustments of a bridge contact H which are effected by means comprisinga thermometer bulb F responsive to the temperature of the heater. Thecontrol system shown in Fig, 3 is also like that shown in Fig. 1, inthat it comprises two bridge circuits IB and IC, different points ofwhich are connected into a control circuit operative through anamplifier EA to control the motor C. Theamplifier EA and associatedmotor control means of Fig. 3 are simpler in type than the amplifier Eand associated control parts D and 2i of Fig. 1, and each of the bridgecircuits IB and IC differs somewhati'rom each of the circuits I and IAof Fig, 1.

The control system shown in Fig. 3 includes a transformer having a,primary coil II connected between supply conductors i and 2, and havingbridge circuit energizing secondaries Ii and I2, and amplifierenergizing secondaries II and N. The secondary winding I energizes thebridge 13 which comprises two slide wire resistors ll and 58, connectedin parallel with one another and in series with a resistor 51 betweenthe terminals of the secondary 5| when a switch I is thrown to its lefthand position for automatic control. The resistor 55 is engaged at avariable point along its length by the control motor slider contact J.The resistor 58 is engaged at a variable point along its length by amanually adjustable slider contact II. A' contact it manually adisconnected by a conductor '2, switch It, and

conductor It to the contact II of the bridge 13, when the switch 83 isturnedinto its left hand position. 'The contacts H and Jet Fig. 3 areconnected by conductors it and It, to the primary winding ll of atransformer", and with the switch N in its left hand, automatic controlposition, the relative positions of the contacts H and J determine theenergizing potential impressed on the primary oi transformer 68 andthereby determine the bias potential impressed on the amplifier EA.

When the switch 83 is shifted into its right hand position, the bridgecircuit 'IC ceases to exert any control function, and bridge IB may thenbe used to manually control the motor by adjusting the contact it alongthe slide wire resistor 58. In such manual control, theamount of gridbias potential impressed on the amplifier EA depends upon the relativepositions of the contacts J and II, the latter being then connected by aconductor it to the same terminal of the transformer coil 81 to whichthe contact H is connected. When the switch I! is adjusted into itsright hand position for manual control operation, itsubstitutes aresistance ll of fixed value for the resistance I! in the energizingcircuit to bridge IB. This makes it practically feasible to adjust thevalve from its fully closed to its wide open position, by a movement ofthe contact 58- from one end to the other of the resistor I. Sinceresistance It is fixed in value, a given adjustment of contact ll alwaysproduces a corresponding adjustment of valve B.

The electronic amplifier EA of Fig. 3 comprises an electronic. valve 1including a pair of triodes within a common envelope. The cathodes ofthe two triodes are connected through an adjustable I macros adjustmentto the fuel valve is. This opening adiustment of the fuel valveterminates when the corresponding adjustment or the contact J issufficient to neutralize the eilect on the motor control circuit of theinitial adjustment of the contact H. If, when the control temperaturefalls below normal, the contact 85 engages the contact II and energizesthe winding ll, when the-fur. nace temperature rises above normal thecontact 85 'will engage the contact 23' and energize the winding 24 and,thereby cause the motor 0 to give a closing adjustment to the valve B.

The control system shown in Fig. 3 includes no provisions for efiectingan automatic reset or compensating action. Automatic reset may read--ily be provided, however, by associating with the control apparatusshown in Fig. 3, a reset motor and bridge circuit IA, in the same mannerin which the motor L and bridge circuit IA of Fig. 1 are associated withcorresponding control parts in F18. 1.

In the form 0! my invention illustrated by way of example in Fig. 4, useis made of three bridge circuits ID, IE and IF, which include as theirrespective energizing elements, the secondary windings 9!, t2 and 03 ofa transformer which has its primary winding it connected between asupply conductors I and 2.- As shown, the three bridge circuits IE, 1])and IF are all alike, each,

including a slide wire resistor 94 and a second resistor 95, connectedin parallel with one another and in series with a resistance 9' betweenterminals of the corresponding transformer secondary SI, 92 or". Eachbridge circuit includes a manually adjustable contact 91 which forms .ameans for short circuiting more or less of the corresponding resistance88.

resistance II to the midpoint of the secondary I winding III of thetransformer it. The winding It has one end connected to one, and has itsother end connected to the second of the two triode control grids E". Arelay winding Ii is included in circuit with the anode E" of one triodesare supplied with anode voltage by the transformer-secondary winding ll,and the sectricde, and a relay coil 82 is included In circuit with theanodev E" of the other triode. Both positionandneitherofthemotorwindingsuand ltiscnergised.whentbecontrolsystemiaunbalanced by a decrease below normal of thefurnace temperature to which the bulb P responds,

. armature l! is tilted to bring the contact it into engagement with oneof the contacts 28' and 281, and thereupon energises the motor 0 foroperation lathedirecticntogivleanopening A control element GE, shown asaBourdon tube, adjusts a slider contact HE along the resistor 94 of thebridge IE, and a second control element GF, also shown as a Bourdontube, adjusts a slider contact HF alongthe resistor 04 or the bridge ID.The regulatorelement CBD includes means collectively corresponding, or

analogous in function to the previously described motor 0, fuel valve Boperated by said motor, and motor control means D and II for said imtor.The midpoint Ila of the resistor 95 of the bridge ID is connected by 'aconductor ll to the contact HE. The resistor midpoint lid of bridge IEis connected by a conductor it to the contact HF, and the resistormidpoint "a of the bridge 11'' is connected by a conductor "I to one ofthe input terminals E' of an amplifier E, which may be like theamplifier E of Figs. 1 and 2. The second input terminal of the amplifierE of Fig. 4 is connected to the contact MD. The output terminais E ofthe amplifier E are connected to the 4 control terminals C of theregulator element CBD, which also includes energizing terminals C"connected between the supply conductors i andl.

The particular control system shown mm. 4

algebraic sum, of the adjustments given the con- 1 ,tscts HE and HF. Forexample, the system of rigimaybeusedforfurnacecontrol withthepressureinthe BourdontubeGEvlwlngwith the temperature at one furnacepoint, and with asaonas the pressure in the Bourdon tube G1" varyin withthe temperature at a second point of the furnace, at which thetemperature normall increases or decreases when the temperature at thefirst mentioned point increases or decreases, respectivel In such casethe eilect of a simultaneous decrease or increase in both temperaturesis to produce a greater corrective valve, or other control, adjustmentthan would be produced by one only of the two temperature changes. On

the other hand, under an abnormal condition in which the temperature atthe one furnace point increases, while-the temperature at the otherpoint decrease the resulting control action, if any, is proportional tothe algebraic sum or the two temperature changes, and prevents theoveradjustment of the fuel valve or other control element in onedirection or the other, which would then occur it the adjustment wereresponsive to the change inone only of the two temperatures.

As will be apparent, the eilect of an adjust- Y merit of contact :1along the resistance 98 of the bridge ID isto change the throttlingrange of the control system, while the efi'ect 01' an adjustment ofeither of the other two contacts 91 to increase or decrease the amountof resistance 96 in the corresponding bridge circuit, respectively, isto decrease or increase the elect of a given change in the position ofthe corresponding control contact HE or HF, on the over-all action orthe control system.

2.Acontrolsystem asspecifiedinclaimlin which each 0! the bridge circuitsincludes a second resistor connected in parallel with the said resistoralong which the slider contact is adjustable and in which a midpoint 01'said second resistor forms the second of the output terminals of thebridge circuits.

3. A control system as specified in claim 1 including adjusting meansindividually associated with each or said bridge circuits for adjustingthe latter and thereby varying the elect produced by a given adjustmentof its slider contact on the current fiow in the control circuit.

4. A control sytem as specified in claim 1 including an electronicamplifier having input taroperating to produce a control eflect and toad- While in accordance with the provisions or the statutes, I haveillustrated and described the best forms of embodiment of my inventionnow known to me, it will be apparent to those skilled in the art thatchanges may be made in the forms of the apparatus disclosed withoutdeparting from the spirit of my invention, as set forth in t e appendedclaims and that in some cases certain i'eatures oi my invention may beused to advantage without a corresponding use of other features.

Having now described my invention, what I claim as new desire to secureby Letters 7 Patent, is:

1. An electric control system comprising in combination a plurality ofseparate bridge circuits each of which has two output terminals andincludes at least one resistor and a slider contact adjustable alongsaid resistor and forming one of said output terminals, said-systemincluding three such resisters and sliders and a fixed output terminal,means connecting, all of said output terminals in a control circuit, areversible control motor controlled by said control circuit andoperating in accordance with the direction of current flow therein toproduce a control eilect and to adjust one or said slider contacts,means for adjusting another oi said slider contacts in accordance withthe direction and just the second or said slider contacts in accordancewith the control eflect produced, a second bridge. circuit including tworesistors connected with the direction and extent oi inparallel and aslider contact adjustable along one of said resistors, means connectingthe midpoint of the second oi the two last mentioned resistors and allof said slider contacts in a control circuit, and means responsive tocurrent flow in a said control circuit controlling the operation of saidcontrol motor, a reversible reset motor and energizing means responsiveto the departure of the value ofsaid control quantity from the normalvalue thereof for operating the last mentioned motor to adjust the saidslider contact or said second bridge circuit when the value of saidextent of the departure of a control quantity at least, of said bridgecircuits for adjusting said 7 bridge circuit and thereby varying theetfect produced bya given adjustment or the slider contact oi the lastmentioned bridge circuit on the current flow in the control circuit,

control quantity diilers from the normal value thereof. a

6.1m electric control system comprising in combination a control motor,three bridge circuits each including a resistor, a slider contactadjustable along said resistor, and'a second resistor connected inparallel with the first mentioned resistor, means for adjusting theslider contact or one or said circuits in accordance with variations inthe value of a control quantity,

-' means for adjustingthe slider contact of another or said circuits inaccordance with variations in the value of a second control quantity,means for adjusting the slider contact of the third circuit inaccordance with the operation or said control motor, and means forconnecting said slider contacts and the midpoint of the second resistorof each or said bridges into a. control circuit, and controlling meansfor said control motor responsive to current flow in said controlcircuit r 7. A control system comprising in combination a. bridgecircuit having two output terminals and including a resistor and aslider contact adjustable along said resistor and forming one of saidterminals, means for adjusting said contact in accordance withvariations in a control quantity, a control motor, a second bridgecircuit including two resistors connected in parallel and two slidercontacts, one of which is manually adjustable along one of the lastmentioned resistors and the other of which is adjustable along the tionto increase the potentialdrop in second of the last mentioned resistorsby and in accordance with the operation of said control motor, a switchadjustable between two positions, means cooperating with said switch inone position of the latter to establish-a control circuit ,including thetwo last mentioned slider contacts and said output terminals, meanscooperating with said switch when the latter is in its second,

position to establish a second control circuit including the lastmentioned slider contacts but 'notincluding said output terminals, meansresponsive to and controlling said control motor in accordance withcurrent flow in each of said control circuits, and adjusting means forsaid second bridge circuit rendered operative by the adjustment of saidswitch into its second posiof said second bridge circuit.

8. An electrical control system comprising in a combination a pluralityof separate bridge cirterminals. one output terminal of another of thebridge circuits being fixed, a reversible control motor, motorenergizing means responsive to current i'low in said control circuit andoperating said control motor in accordance with the direction of currentilow in said control circuit to prothe resistors asoonos' adjustment oithe slider contact of said other one 01' said bridge circuits on thecurrent iiow in the control circuit without aiiecting the elect-oi agiven adjustment of either oi the contacts or said one bridge circuit onthe current flow in the control circuit.

9. An electrical control system in combination a plurality oiseparatebridge circuits each of which has two output terminals and includes aresistor and a,slider contact adjustable along said resistor. andforming one of said output terminals, means connecting all oi saidoutput terminals in series in a control circuit, one of said bridgecircuits including a second resistor in parallel with said resistor anda slider contact adjustablealong said second reslstor,said lastmentioned slider contact forming one of said output terminals, oneoutput terminal oi another of the bridge circuits being fixed, areversible duce a control ei'i'ect and to adjust one or the slidercontactsot said one bridge circuit, means for adjusting the other ofsaid slider contacts of said one bridse'circuit in accordance with thedirection and extent oi the departure of a con-' trol quantity from anormal value of the latter,

means for adjusting the slider contact 01 the other one of said bridgecircuits, and adjustin 7 means individually associated with said otherone of said bridge circuits-tor adjusting said bridge circuit andthereby varying the eii'ect or a given control motor, motor energizingmeans responsive to current flow in said control circuit'and operatingin accordance with the directioneof current new in said control circuitto produce a control eifect and to adjust one of the slider contacts 01said one bridge circuit, means for adjusting the other 01' said slidercontacts of said 'one bridge circuit in accordance with the directionand extent or the departure of a control quantity from a normal valueoithe latter. a reversible reset motor, energizing means for said resetmotor responsive to the "departure oi' the control quantity from thenormal value thereof and operating said reset motor to adjust the slidercontact of the other of said bridge circuits when the value of saidcontrol quantity diilers from the normal value thereof, and adjustingmeans individually associated with said other one of said bridgecircuits for adjusting said bridge circuit and thereby varying theeil'ect of a given adjustment of the slider contact of said other one ofsaid bridge circuits on the current flow in the control circuit withoutaiiecting the eifect of a given adjustment oi either of the slidercontacts of said one bridge circuit on 4 the current flow in thecontrolcircuit.

HARRY 8. JOHN.

